Wire bonding is typically applied to make electrical connections between an integrated circuit die or chip and a carrier on which the die is mounted. Bonding wires are attached to bond pads on the chip and bonding leads on the carrier respectively by ultrasonic welding using an ultrasonic transducer which is integrated into a wire bonding apparatus. The ultrasonic transducer is an energy-converting device which converts electrical energy into ultrasonic vibrations and transmits the ultrasonic vibrations to a capillary at a tip end of the transducer to perform wire bonding.
FIG. 1 illustrates a side view of a conventional wire bonding apparatus 10 with a transducer 12 positioned over a bonding area for wire bonding. At one end of the transducer 12 is a capillary 13 for wire bonding. The other end of the transducer 12 is connected to an ultrasonic generator 14 which is housed in a transducer holder 16 mounted on a bond arm 18. The bond arm 18 is connected to a sliding bar 22 at a pivot 20. The sliding bar 22 may move and position the transducer 12 along the x and y axes. The bonding arm 18 is also mounted to a bonding stage 24. The bonding stage 24 moves along the x and y axes together with the sliding bar 22. An optical system 26 is further mounted on the bonding stage 24 such that the optical system 26 moves along the x and y axes together with the sliding bar 22 over a wire bonding platform.
A carrier 28, on which components such as semiconductor dice and bonding wires are attached, is held in position by a window clamp 30 on the wire bonding platform for wire bonding to be performed. The wire bonding platform includes a heater block 32 which provides heat to the carrier 30 to facilitate wire bonding conducted on it.
A camera 27 in the optical system 26 captures an image of a die on the wire bonding platform so that the position of the die on the wire bonding platform can be observed for the purpose of wire bonding. As heat is transmitted to the immediate vicinity of the heater block 32, and the optical system 26 is positioned over the heating zone during wire bonding, the optical system 26 is exposed to heat from the heater block 32 by radiation and by convection. The heat may cause the optical system 26 to expand and increase in length. This changes the distance of a light path to the camera 27 thus affecting the determination of the position of the die on the wire bonding platform. As a result, wire bonding may be inaccurate.
FIG. 2 shows the optical system 26 of FIG. 1 moving away from the heating zone. As a result, the optical system 26 is exposed to less heat from the heating zone. The optical system 26 may then cool and contract, thereby changing the distance of the light path to the camera 27. This again results in inaccurate determination of the position of a die on the wire bonding platform and therefore inaccurate wire bonding.
In view of the above, maintaining the thermal stability of the optical system 26 is essential to perform wire bonding accurately. This is particularly so when very fine wires and bond pitches are involved.